A plurality of systems uses a battery implemented as a battery pack or battery array, including a plurality of battery cells connected in series with each other.
When such a battery cell is charged to a much higher voltage or a much lower voltage than the voltage within a rated charge range, it may be dangerous.
Further, imbalance in the charged state of battery cells is caused by various factors, and occurs during the manufacture of batteries or the charge or discharge of batteries. In particular, in the case of lithium ion cells, the manufacture of cells is strictly controlled within a company to minimize the differences between the capacities of the cells of a battery array. However, imbalance or inequality between cells may occur due to various factors, regardless of the states of the cells, in which balance or equality is maintained after the cells are initially manufactured.
The factors influencing the imbalance of cells may include, for example, the chemical reactions, impedances and self-discharge rates of respective cells, reduction of the capacities of the cells, variation in the operating temperatures of the cells, and other types of variation between the cells.
Inconsistency in the temperature of cells is an important factor responsible for causing imbalance in cells. For example, “self-discharge” is caused in a battery cell, and is a function of a battery temperature. A battery having a high temperature typically has a self-discharge rate higher than that of a battery having a low temperature. As a result, the battery having a high temperature exhibits a lower charged state than the battery having a low temperature, with the passage of time.
Imbalance is a very series problem in the charged state of a battery. For example, this problem may typically occur in electric vehicles, and the capability of a battery to supply energy is limited by the battery cell having the lowest charged state.
If this battery cell is consumed, other battery cells lose the ability to continue to supply energy. This is the same even if the other battery cells still have the ability to supply power. Therefore, an imbalance in the charged state of battery cells reduces the power supply capability of the battery.
Of course, the above description does not mean that when one or more battery cells are consumed the supply of power by the remaining battery cells is completely impossible. However, it means that, only in the case of series connection, even if one or more battery cells are fully consumed, the battery can be continuously used as long as charge remains in the remaining battery cells, but, in that case, voltage having a reversed polarity is generated in the battery cell which has been fully discharged, and, as a result, the battery cell may be in danger of explosion due to the overheating thereof, or due to the generation of gas, and thus the battery loses power supply capability.
Various methods of correcting imbalance in the charged states of battery cells have been proposed, and one of the methods is shown in FIG. 1.
FIG. 1 is a diagram showing a conventional charge equalization apparatus.
Referring to FIG. 1, the conventional charge equalization apparatus includes a transformer T, control switches SW1 to SWn, and a voltage detection and drive signal generation unit 10.
The transformer T is configured such that it includes N primary windings and a single secondary winding, the N primary windings are connected to a common core, the primary windings and the secondary winding have different polarities, in other words, the dots of the primary windings and the dot of the secondary winding are placed on different sides, the N primary windings have the same number of turns, and a turns ratio of the primary windings to the secondary winding is N1:N2.
In the transformer T, the N primary windings are connected in parallel with N series-connected batteries B1 to Bn, respectively, and a diode D is connected between the secondary winding and the first battery B1, among the N series-connected batteries B1 to Bn, so as to prevent energy from being supplied by the N series-connected batteries B1 to Bn to the secondary winding.
The control switches SW1 to SWn are respectively connected between the second ends (terminals on which dots are not formed) of the primary windings of the transformer T and the anodes (−) of the batteries B1 to Bn, and are configured to form closed loops so as to supply energy from the batteries B1 to Bn to the primary windings of the transformer T1 in response to a drive signal provided by the voltage detection and drive signal generation unit 10.
The voltage detection and drive signal generation unit 10 detects the voltages of respective series-connected batteries B1 to Bn, compares the detected voltages with a reference voltage, and thus generates a drive signal required to discharge batteries charged to voltages greater than the reference voltage, that is, overcharged batteries.
The above-described charge equalization method, performed by the conventional charge equalization apparatus, is described in detail below.
First, the voltage detection and drive signal generation unit 10 detects the voltages of respective N series-connected batteries B1 to Bn.
As a result of the voltage detection, when it is determined that a charge imbalance exists between the N series-connected batteries B1 to Bn, the voltage detection and drive signal generation unit 10 simultaneously turns on all of the control switches SW1 to SWn.
Then, a charge automatically moves from a battery having a high voltage to a battery having a low voltage during the time for which the control switches SW1 to SWn are turned on, thus realizing charge equalization. Further, when the control switches SW1 to SWn are simultaneously turned off, energy stored in the magnetizing inductors of all of the primary windings is recharged in the N series-connected batteries B1 to Bn through the rectifying diode D on the secondary side.
In this way, the charge equalization apparatus of FIG. 1 realizes charge equalization because charge moves due to the difference between the voltages of the N series-connected batteries B1 to Bn.
Meanwhile, a lithium ion battery is disadvantageous in that, even if variation is present in the State of Charge (SOC) between respective batteries, the voltage difference is very small, and thus little charge movement occurs. Accordingly, the conventional charge equalization apparatus is problematic in that, when N lithium ion batteries are connected in series with each other, the charge equalization characteristics of the batteries are deteriorated.
Further, the conventional charge equalization apparatus is problematic in that, since a number of primary windings corresponding to the number of batteries is coupled to a single common core, it is difficult to manufacture a transformer when the number of batteries increases.
Furthermore, the conventional charge equalization apparatus is disadvantageous in that, as the number of batteries increases, the voltage stress on a diode for providing a current path for the magnetizing current so as to prevent the saturation of the transformer increases.